Answer:
B (?)•The dog flung itself into the swimming pool.
Step-by-step explanation:
definition of a complex sentence:
A complex sentence combines a dependent clause with an independent clause. When the dependent clause is placed before the independent clause, the two clauses are divided by a comma; otherwise, no punctuation is necessary.
"The dog flung itself." is an independent clause because it expresses a complete thought, this alone could be considered a sentence.
"into the swimming pool." is a dependent clause, it doesn't express a complete thought and on its own is very unclear;
however, by putting the two together you -- by definition-- get a complex sentence, no comma is needed because the independent clause is used first.